The invention is based on a sensorless, brushless electric motor that is used in a motor vehicle as an actuator for flaps, such as air flaps or spoilers. For this kind of actuator, it is desirable if the electric motor is operated at a constant rotational speed. For this purpose, the electric motor preferably has a control having pulse width modulation (PWM).
Depending on the position of the air flaps, it so happens that the weight that acts on the air flaps reinforces the movement of the motor. In order to achieve a desired rotational speed, a lower motor current is then required.
However, this could also mean that the force of weight alone, without any current, drives the motor above the desired rotational speed. In this case, the motor would have to be actively braked so as to attain the nominal rotational speed. This can be achieved by operating the motor as a generator.
On braking, several measured variables of the motor change their sign and can thus no longer be measured by unsophisticated electronics, for example, which is why the motor can no longer be effectively controlled. The integrated zero crossing identification, which is necessary for sensorless commutation, may also be thereby impaired.
Due to the braking operation, negative phase currents occur, for example, that could entail undesirable disturbances and effects. Additionally, any switching spikes that may occur have to be filtered, which makes the control circuit more complex and expensive.
The negative current cannot be measured by the available current measuring device. In addition, overvoltages may possibly occur in the internal power supply that could cause damage.
Moreover, care must be taken to prevent the braking energy produced in generator mode from being fed back into the vehicle electrical system. Instead, it has to be dissipated which means additional, more complex circuitry has to be provided.
Regulation of the rotational speed in the braking operation is thus only made possible using additional measurement technology that can also process negative currents, which, however, is complex and expensive.
An object of the invention is to create a method for controlling this kind of actuator in which the above-mentioned problems do not occur and which can also be regulated to a nominal speed even during the braking operation.